This invention relates to concentrated, stable solutions of alkali alkoxides of secondary and tertiary alcohols, to a process for the production thereof and to the use thereof.
Alkali metal alkoxides Rxe2x80x94OM (R=alkyl having 3 to 20 C atoms, M=Li, Na, K, Rb, Cs) are compounds sensitive to hydrolysis which, by virtue of the basic properties thereof, are frequently used in organic synthesis. They are accordingly used as condensation, esterification and alkylation agents and for introducing the alkoxy group into other compounds (Williamson synthesis).
Alkali alkoxides are generals prepared by the action of alkali metals on alcohols in accordance with
Rxe2x80x94OH+Mxe2x86x92Rxe2x80x94OM+xc2xdH2↑xe2x80x83xe2x80x83(1).
Apart from the elemental metal (M=Li, Na, K, Rb, Cs), it is also possible to use other reactive metal compounds (xe2x80x9cmetalation agentsxe2x80x9d), such as alkali metal hydrides, amides and organo-alkali metal compounds (for example butyllithium).
The reaction generally proceeds in the liquid phase, i.e. in the presence of a solvent. In the case of alkoxides of primary alcohols (for example methanol, ethanol, n-butanol), the solvent is preferably the primary alcohol itself. The alkoxides of secondary and tertiary alcohols are generally only sparingly soluble in alcohols. They are accordingly frequently produced in aprotic solvents (for example hydrocarbons, ethers).
After evaporation of the synthesis solutions, the alkoxides may be sold in the form of solid (pulverulous) products. The disadvantage of this commercial form is that, due to their basic properties, alkoxide dusts have a strongly corrosive action, i e. appropriate protective measures must be taken in order to prevent physical contact with the alkoxide powders. Liquid delivery forms, i. e. solutions, are particularly desired in order to avoid this handling-related disadvantage. Solutions are, however, only of economic interest if the alkoxide has good solubility, i. e. greater than 20%, over a wide temperature range.
The individual solubilities of alkoxides in a solvent are primarily determined by the metal M and the alkyl residue. In general, alkoxides derived from lithium are most readily soluble. Solubility is also a function of the volume and bulkiness of the alkyl group; the xe2x80x9clargerxe2x80x9d said group, the better is solubility in preferably slightly polar solvents (for example hydrocarbons).
However, in addition to these parameters influenced by the particular alcohol and metal, there are other factors which influence individual solubility. In the case of alkali alkoxides derived from secondary and tertiary alcohols, the residual alcohol content is one such factor.
It is known that alkoxides form complexes with free alcohols, which complexes are sparingly soluble in a polar or slightly polar solvents.
xRxe2x80x94OM+yRxe2x80x2xe2x80x94OHxe2x86x92(Rxe2x80x94OM)xxc2x7(Rxe2x80x2xe2x80x94OH)y↓xe2x80x83xe2x80x83(2)
The presence of free alcohol in alkoxide solutions results either from incomplete reaction according to equation (1) or is a consequence of hydrolysis due to contact with air and/or water according to equation (3):
Rxe2x80x94OM+H2Oxe2x86x92Rxe2x80x94OH+MOHxe2x80x83xe2x80x83(3)
The applicant""s own investigations revealed that, for example in the case of concentrated sodium tert.-butylate solutions in methyl tert.-butyl ether (MTBE), tetrahydrofuran (THF) or toluene, the xxe2x80x2y stoichiometry of the sparingly soluble complex formed according to equation (2) is approx. 3-6:1, i. e. free alcohol is capable of removing a distinctly above stoichiometric quantity of alkoxide from the solution. The applicant""s own investigations revealed that, for example in the case of concentrated sodium tert.-butylate solutions in methyl tert.-butyl ether (MTBE), tetrahydrofuran (THF) or toluene, the x:y stoichiometry of the sparingly soluble complex formed according to equation (2) is approx. 3-6:1, ie. free alcohol is capable of removing a distinctly superstoichiometric quantity of alkoxide from the solution.
From the above, it may be concluded that solutions of alkali alkoxides derived from secondary and tertiary alcohols should contain as little water and alcohol as possible in order to achieve maximum alkoxide solubilities.
The object of the present invention is to provide away of increasing the solubility of alkali metal alkoxides derived from secondary and tertiary alcohols and of reducing the susceptibility thereof to hydrolysing conditions, so improving the stability of these alkali metal alkoxide solutions.
The object is achieved by the solutions stated in claim 1, while claims 2 to 9 state variants of the solutions according to the invention. Claims 9 to 13 state a process for the production of the solutions according to the invention and claim 14 states a use of the solutions according to the invention.
The concept of the invention is to dissolve a mixture of an alkoxide ROM and an alkali metal hydroxide Mxe2x80x2OH in an aprotic solvent, wherein R is a secondary or tertiary alkyl residue having 3 to 20 C atoms and wherein M and Mxe2x80x2 are mutually independently Li, Na, K, Rb or Cs.
Secondary or tertiary alcohols are, for example, isopropyl alcohol, sec.-butyl alcohol, tert.-butyl alcohol or tert.-amyl alcohol.
It has been found that a hydroxide content in the range between at least 0.5 and at most 15 mol % (preferred ranges are disclosed in the claims) in order to exert a distinctly favourable effect on the solubility of the alkali alkoxides. For applications in organic synthesis, such a hydroxide content is not generally disruptive because alkali hydroxides are less basic than alkoxides, ie. different reaction results should not generally be anticipated.
In order to prevent sparingly soluble alcohol/alkoxide complexes from precipitating out, the residual alcohol content should be as low as possible. The maximum residual free alcohol content of the solutions is at most 1.0 mol %, preferably at most 0.5 mol %.
Solvents which may be used are polar and/or non-polar aprotic solvents. It is possible to use aromatic hydrocarbons (for example toluene, benzene, xylene, ethylbenzene), or open-chain or cyclic aliphatic hydrocarbons (for example pentane, hexane, cyclohexane, heptane, octane) or ethers (open-chain or cyclic, mono- or polyfunctional, for example diethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, methyl tert.-butyl ether, 1,2-dimethoxyethane (1 ,2-DME), diethylene glycol dimethyl ether) or amides (for example dimethylformamide DMF) or dimethyl sulfoxide DMSO or acetals (for example diethoxymethane or diethoxyethane) or nitrites (for example acetonitrile). Mixtures of these substances may also be used.
There are various manners in which the alkali metal hydroxide may be incorporated: Isolated, pulverous alkoxides may be combined and mixed with an alkali metal hydroxide and then dissolved in an anhydrous, aprotic solvent. Should the hydroxide not be in finely divided form, the mixture would have to be ground. Since commercially available hydroxides are not completely anhydrous (they are markedly hygroscopic), it is often impossible to avoid entraining small quantities of water in this manner which have a negative impact on solubility. This may be counteracted by post-drying the solution (addition of Mxe2x80x2, Mxe2x80x2H, molecular sieve, azeotropic removal of water).
It is simpler and more effective to incorporate the hydroxide during synthesis of the alkoxide. This is achieved, for example, by apportioning the calculated quantity of hydroxide (Mxe2x80x2OH) or of water to the metalation agent (i.e. the alkali metal itself or the hydride, amide or an organoalkali metal compound). The solvents used are the above-mentioned polar and/or non-polar aprotic solvents. One particularly elegant variant also consists in adding, instead of completely anhydrous alcohol, a water/alcohol mixture prepared in the calculated ratio. When using water for in situ production of alkali metal hydroxide, an appropriate additional quantity of metalation agent must initially be introduced.
The person skilled in the art is aware of the various processing variants for alkoxide synthesis. It is, for example, also possible initially to introduce the alcohol and to apportion the metalation agent thereto. There are numerous conceivable variants as to how and when the water and metal hydroxide are incorporated.
In all cases, it is vital for both the alcohol and the water to be consumed as completely as possible in the reaction with the metalation agent. Consequently, depending upon the process variant, the metalation agent should preferably be used in excess (approx. 1 to 50%, preferably 1 to 10%).
It has been found that the presence of small quantities of water (0.5 to 10 mol %) generally has no significant effect upon the course of the formation reaction (i.e. rate of reaction, selectivity etc.). Accordingly, no further measures deviating from the prior art need be taken.
The alkoxide solutions according to the invention are used as reagents in organic synthesis.
The advantage of the secondary and tertiary alkoxide solutions according to the invention is the higher concentration of the solutions in comparison with the prior art, combined with the simultaneous stability of the solutions, which is advantageous in processing and economic terms.